Method and device for verifying a group of non-volatile memory cells

ABSTRACT

The invention relates to a method of simultaneously checking a first electrical state of a group of N cells of a non-volatile memory, characterized in that it comprises the following steps of simultaneously selecting in readout the N memory cells (CE) to check as well as a checking cell (Cveri), summing the N read signals to obtain a summed signal,. and comparing the summed signal with the signal read on the checking cell (Cveri) to provide a given state is signal when the summed signal is less than the signal read on the checking cell (Cveri), indicating that the N memory cells are in the electrical state and another electrical state signal indicating that at least one memory cell is not in the electrical state when the summed signal is greater than the signal read on the checking cell.

[0001] The invention relates to memories in integrated circuit form and, more particularly, those of the non-volatile type known under the acronyms EPROM for Electrically Programmable Read Only Memory, EEPROM for Electrically Erasable and Programmable Read Only Memory and FLASH for an EEPROM type memory which is erasable by groups of memory cells.

[0002] In these non volatile memories, the invention concerns a method and device allowing to control a group of memory cells to check whether they are all in the same state.

[0003] Such a check is necessary in many circumstances, for instance in memory manufacturing test operations or during use of the memory in an electronic system such as a microprocessor.

[0004] At present, such a check is carried out cell by cell by selecting each cell in a readout mode and comparing the read signal with a reference signal supplied by a reference cell which is identical to the memory cell. Such a checking process is long and laborious.

[0005] Accordingly, an object of the present invention is to implement a process and to provide a device for checking the state of cells of an integrated circuit memory which is fast.

[0006] This object is achieved by performing a simultaneous readout of the memory cells in groups of N cells, by summing the N read signals and by comparing their sum to a reference signal supplied by a reference cell so as to determine whether the N memory cells of the group are in the same state.

[0007] The invention therefore concerns a method of simultaneously checking the electrical state of a group of N memory cells in a memory of the non-volatile type, where each cell of the memory can take on that electrical state or an other electrical state, the memory further comprising at least one memory cell, designated checking cell, which is in the other electrical state but exhibiting a characteristic curve in readout which is modified relative to the other cells of the memory,

[0008] characterized in that it comprises the following steps:

[0009] (a) simultaneously selecting in readout the N memory cells to check as well as the checking cell,

[0010] (b) summing the N read signals to obtain a summed signal,

[0011] (c) comparing the summed signal with the signal read on the checking cell to provide a given state signal when the summed signal is less than the signal read on the checking cell, indicating that the N memory cells are in the electrical state, and another state signal indicating that at least one memory cell is not in the electrical state when the summed signal is greater than the signal read on the checking cell.

[0012] The invention also relates to a device for simultaneously checking the electrical state of a group of N memory cells in a memory of the non-volatile type, where each memory cell can take on that electrical state or an other electrical state,

[0013] characterized in that it further comprises:

[0014] at least one memory cell, designated checking cell, which is in the other electrical state but exhibiting a characteristic curve in readout which is modified relative to the other cells of the memory,

[0015] means for simultaneously selecting in readout the N memory cells to check of the group as well as the checking cell,

[0016] means for summing the N signals read in the N memory cells to obtain a summed signal, and

[0017] comparison means for comparing the summed signal with the signal read in the checking cell, the comparison means supplying a first state signal when the N memory cells are all in the same above-mentioned electrical state and a second state signal when just one memory cell among N is in that other electrical state.

[0018] Other characteristics and advantages of the present invention shall become more apparent from reading the following description of a specific embodiment, the description being given with reference to the appended drawings in which:

[0019]FIG. 1 is a simplified diagram of a device for checking a group of memory cells in accordance with the invention,

[0020]FIG. 2 is a diagram showing memory cells of a part of the memory 10 of FIG. 1,

[0021]FIG. 3 is a diagram of curves showing the variation of current drain versus gate voltage as a function of the state of the memory state.

[0022] The diagram of FIG. 1 shows a memory 10 whose memory cells CE are organized in n lines numbered L1 to Ln and r columns numbered C1 to Cr, each memory cell CE being at the intersection of a line L and a column C as shown by the diagram of FIG. 2 for two lines L1 and L2 and three columns C1, C2 and C3.

[0023] Each memory cell CE comprises (FIG. 2) a MOSFET transistor, e.g. of the insulated gate N-type, of which the source S is connected to ground, the drain is connected to a column C1, C2 or C3 and the gate G is connected to a line L1 or L2.

[0024] The memory 10 comprises an additional column REF comprising an additional memory cell Cref at each line L1 to Ln (Cref1 to Crefn) to provide a reference memory cell when reading another cell of the same line. Indeed, it is the comparison between the signal read on a memory cell CE and the one read on the reference cell Cref of the same line which determines the binary content “0” or “1” of the cell.

[0025] The selection of a line and a column, and hence a cell CE, is obtained respectively by address decoding circuits 12 and 14 which each cooperate respectively with a power supply circuit 16 and 18 so as to apply the appropriate voltage values to write into and read from the memory cells.

[0026] The output terminals of all the columns are each connected to an input terminal of a readout amplifier illustrated collectively by circuit 20.

[0027] The output terminals of the amplifiers 20 corresponding to columns C1 to Cr are connected to a same input terminal BC of a comparator 22, while the other input terminal Bref is connected to the output terminal of the amplifier corresponding to reference column REF.

[0028] The address signals and other control signals of the memory 10 are supplied in a known manner by a microprocessor or microcontroller 24, the latter possibly being part of a test system external to the memory 10 or being intended to operate directly with that memory.

[0029] The operation of the memory 10 shall now be briefly described in conjunction with the diagrams of FIG. 3. The drain/source current Id of a memory cell varies as a function of the voltage VG applied to the gate G according to the curve EFFA for a binary digit “1”, and according to the curve PROG for a binary digit “0”. In the case of a FLASH memory, and by convention, curve EFFA corresponds to an erasure of the cell while curve PROG corresponds to a write (programming) operation.

[0030] The intermediate curve in dotted line A belongs to a reference cell Cref and corresponds to that of a memory cell CE in the erased state but with a different gain value at the output, less than one, that gain being obtained by amplifier 20 connected to the reference column REF.

[0031] During a read operation, there is applied a gate voltage VG1 comprised between the two curves EFFA and PROG, and the current Id1 of the cell CE and the current Iref of the reference cell Cref are measured:

[0032] if Id1 is less than Iref, the memory cell is at state “0” according to the adopted convention (PROG curve),

[0033] if Id1 is greater than Iref, the memory cell is at the “1” state (EFFA curve).

[0034] This comparison is performed for each memory cell of the selected line by the successive selection of columns using address circuit 14.

[0035] In this type of memory, prior to recording “0” and “1” binary information therein, all the cells are “tipped” into the same state, for example the “0” state corresponding the PROG curve of FIG. 3. Accordingly, when a memory cell is to store the binary digit “0”, no operation is performed. Conversely, to store the binary digit “1”, operations are performed to bring the cell into the state of curve EFFA.

[0036] For proper operation of the system, it is fundamentally important that all the cells be in the same “0” state prior to storing information. At present, a check is performed by reading cell by cell.

[0037] The invention proposes a method and device for carrying out this check by groups of cells.

[0038] To this end, the invention selects for readout the cells to check by groups of N, where N can be variable according to requirements.

[0039] To this end, the address/decoding circuits 12 and 14 are provided to select several lines (p) and several columns (q) such that p×q=N, upon receiving a signal “VERI” supplied by the microcontroller 24 on an input terminal 12 ₁ and 14 ₁.

[0040] The N read signals are summed and their sum is compared in comparator 22 with the signal read in a checking cell Cveri.

[0041] In the case where just one line is selected for checking, the checking cell can be the reference cell at the intersection of the reference column REF with the selected line.

[0042] However, the checking cell is preferably a cell provided for that purpose and which is formed in the memory 10. In the diagram of FIG. 1, this cell is represented under the reference Cveri and is selected by the signal VERI. It is connected to the input terminal of the comparator 22 via a two-position switch 30. In the absence of the signal VERI, the switch is in position (a) to connect the column REF of readout reference cells to the input terminal Bref of the comparator. In the presence of the signal VERI, the switch passes to position (b) and connects that input terminal Bref of the comparator to the checking cell Cveri.

[0043] There is a signal VERI for each grouped selection of memory cells CE, each selection corresponding to a number N1, N2, N3, . . . , of memory cells.

[0044] Each signal VERI is applied to a different input terminal 12 ₁, 12 ₂, 12 ₃ or 14 ₁, 14 ₂, 14 ₃ of the address circuits 12 and 14 respectively, only the input terminal 12 ₁ and 14 ₁ having been shown on FIG. 1 for more clarity. On the other hand, there is just one input terminal for the signal VERI on switch 30.

[0045] Comparator 22 supplies a first state signal when the sum of the read signals is less than Iref (FIG. 3), which corresponds to the N memory cells being in the same PROG state. On the other hand, it supplies a second state signal when the sum of the N read signals is greater than Iref, which corresponds to at least one memory cell being in the EFFA state, i.e. the N memory cells selected are not all in the PROG state.

[0046] The invention is based on the fact that the readout current from a cell in the PROG state is very small, on the order of a pico-amp, so that the sum of the N currents is much less than Iref, while the current read in just one cell in the EFFA state is greater than Iref.

[0047] However, the number N and, more generally, the numbers N1, N2, N3 should be chosen such that in the course of the memory's lifetime, the sum of the N currents read in the programmed cells is always much less than Iref.

[0048] It is the memory manufacturer who determines the numbers N1, N2, N3 as well as the arrangement of memory cells that shall be selected by the choice of the number p of lines among n and the number q of columns among r.

[0049] To this end, the manufacturer modifies the address circuits 12 and 14 to allow this selection by groups of N1, N2 and N3 cells from the different VERI signals, one being used per group. 

1. Method of simultaneously checking the electrical state of a group of N memory cells (CE) in a memory (10) of the non-volatile type, where each cell (CE) of the memory can take on said electrical state or an other electrical state, said memory further comprising at least one memory cell, designated checking cell (Cveri), which is in said other electrical state but exhibiting a characteristic curve in readout which is modified relative to the other cells of the memory, characterized in that it comprises the following steps: (a) simultaneously selecting in readout the N memory cells (CE) to check as well as the checking cell (Cveri), (b) summing the N read signals to obtain a summed signal, (c) comparing the summed signal with the signal read on the checking cell (Cveri) to provide a given state signal when the summed signal is less than the signal read on the checking cell (Cveri), indicating that the N memory cells are in said electrical state, and another electrical state signal indicating that at least one memory cell is not in said electrical state when the summed signal is greater than the signal read on the checking cell.
 2. Device for simultaneously checking the electrical state of a group of N memory cells in a memory (10) of the non-volatile type, where each memory cell (CE) can take on said electrical state or an other electrical state, characterized in that it further comprises: at least one memory cell, designated checking cell (Cveri), which is in said other electrical state but exhibiting a characteristic curve in readout (A) which is modified relative to the other cells of the memory, means for simultaneously selecting in readout the N memory cells (CE) to check of said group as well as the checking cell (Cveri), means for summing the N signals read in the N memory cells to obtain a summed signal, and comparison means (22) for comparing the summed signal with the signal read in the checking cell (Cveri), said comparison means supplying a first state signal when the N memory cells are all in the same said electrical state and a second state signal when just one memory cell among N is in that other electrical state.
 3. Device according to claim 2, characterized in that the number N can take on different values (N1, N2, N3), and in that the means for simultaneously selecting in readout the memory cells comprise address circuits adapted to perform this selection and controlled by a signal (VERI) specific to each value of N.
 4. Device according to claim 2 or 3, characterized in that it further comprises a switch (30) connected at one end to an input terminal (Bref) of the comparison means (22) and at the other end, either to the checking cell (Cveri) for checking (position (b)), or to a reference cell (Cref) for classic readout cell by cell (position (a)). 